This invention relates to rotary cone bits used for subterranean drilling and, more particularly, to rotary cone bits having functionally-engineered composite surfaces, and methods for forming the same, having improved mechanical properties of wear resistance and toughness when compared to conventional rotary cone bits.
Cemented tungsten carbide, such as WCxe2x80x94Co is well known for its mechanical properties of hardness, toughness and wear resistance, making it a popular material of choice for use in such industrial applications as mining and drilling where its mechanical properties are highly desired. Because of its desired properties, cemented tungsten carbide has been the dominant material used on rotary cone rock bit surfaces exposed to wear, e.g., on cutting inserts used with rotary cone rock bits. The mechanical properties associated with cemented tungsten carbide and other cermets, especially the unique combination of hardness, toughness and wear resistance, make these materials more desirable than either metals or ceramics alone.
It is known in the art that for cemented tungsten carbide, fracture toughness is inversely proportional to hardness, and wear resistance is proportional to hardness. Accordingly, when using cemented tungsten carbide as a wear surface one must balance the demand for high wear resistance with the desire to have an acceptable degree of fracture toughness. A cemented tungsten carbide material having a high degree of wear resistance may not provide a sufficient degree of fracture resistance for drilling applications, resulting in a wear surface that is brittle and thus susceptible to gross brittle fracture. A cemented tungsten carbide material having a high degree of fracture resistance, while not being brittle and having acceptable impact resistance, may not have a suitable degree of wear resistance for drilling applications.
A known approach for addressing this issue of competing desired properties has been to use a cemented tungsten carbide substrate, and place a cemented tungsten carbide material over the substrate to provide a relatively-more wear resistant surface thereon. In this approach, the cemented tungsten carbide placed on the substrate is specially formulated to provide a greater degree of wear resistance than that of the underlying substrate, and the substrate is formulated to provide a greater degree of fracture toughness than the surface layer. The cemented tungsten carbide used as the wear surface is bonded to the substrate and consolidated by the process of liquid phase sintering.
A known limitation with this approach, however, is that the interface between the substrate and the surface layer must be flat or planar. Thus, this approach is not useful for addressing the need to provide a wear surface formed from cemented tungsten carbide, having both a desired degree of wear resistance and fracture toughness, on a substrate having an irregular or nonplanar interface surface, e.g., an interface surface having a variable or constant radius of curvature.
A further known limitation with this approach is the reliance upon liquid phase sintering to bond the cemented tungsten carbide substrate and surface layer together. During the process of liquid phase sintering it is known that the ductile metal component, e.g., cobalt metal, liquefies and migrates across the boundary or interface between the substrate and surface layer. This migration is not desired because it reduces the intended differential between the two material compositions across the interface, causing the interface to become homogeneous and the related differential material properties to be minimized or eliminated. For example, during liquid phase sintering the cobalt metal constituent in the substrate can migrate into the surface layer, where less of the cobalt metal constituent is desired to provide the desired degree of wear resistance. In this instance, such migration causes an undesired reduction in the wear resistance provided by the surface layer. Thus, this phenomena of liquid phase migration is known to limit the ability to control surface layer properties by use of a material differential approach.
Cemented tungsten carbide constructions known in the art are typically formed into the shape of a green part in sheet form that is sintered to an underlying substrate during the above-described liquid phase consolidation process. The above-described process of forming the green part and the finally-sintered product both limits the types of constructions that can be used to form the final product, e.g., constructions comprising complex microstructures or multiple layers may be outside the scope of practical manufacturing capabilities, and limits the types of products that can include the complex construction, e.g., products having an irregular shape or a nonplanar substrate surface (such as those developed by residual stress analysis), may also be outside of the scope of practical manufacturing capabilities. In many rotary cone rock bit applications, it is desired that a portion of the bit or cutting element having a nonplanar surface comprising a layer of cemented tungsten carbide disposed thereon for purposes of improving wear resistance and fracture toughness at that location.
It is, therefore, desired that functionally-engineered composite surfaces, for use with rotary cone rock bits, be prepared according to principles of this invention in a manner that does not adversely impact the physical properties of either the substrate or the surface material, e.g., in a manner that avoids ductile phase metal migration, when compared to wear resistant surfaces applied by liquid phase sintering method. It is desired that such functionally-engineered composite surfaces be formed in a manner that permits use on substrates having irregular or nonplanar interface geometries. It is further desired that functionally-engineered composite surfaces of this invention provide an improved degree of wear resistance and fracture toughness when compared to conventional cemented tungsten carbide surfaces formed using liquid phase sintering methods.
Functionally-engineered composite wear surfaces, prepared according to principles of this invention, are provided on cutting elements used with rotary cone rock bits, and can be specially engineered to provide a desired degree of wear resistance and/or fracture toughness necessary to meet particular drilling applications. Cutting elements comprising functionally-engineered composite wear surfaces of this invention are formed in a manner that both avoids unwanted material migration, between the wear surface and substrate, and that permits application on nonplanar, e.g., curved, interface surfaces, thereby enabling placement of wear surfaces where not before practical.
As mentioned above, functionally-engineered composites of this invention are used with rotary cone bits that comprise a bit body having at least one leg extending therefrom, and a cone that is rotatably disposed on the leg. The cone typically comprises a plurality of cutting elements that project outwardly therefrom. The cutting elements may comprise a cermet material selected from the group consisting of refractory metal carbides, nitrides, borides, carbonitrides and mixtures thereof.
A functionally-engineered material is disposed over a surface portion of at least one of the cutting elements to form a wear resistant surface thereon. The wear resistant surface has a hardness that is different than that of the underlying cutting element. The wear resistant surface is formed by forming a conformable material mixture by combining one or more powders selected from the group consisting of cermets, carbides, borides, nitrides, carbonitrides, refractory metals, Co, Fe, Ni, and combinations thereof, with an applying agent.
As used herein, the term xe2x80x9cconformablexe2x80x9d is used to describe the nature of the mixture as being in a physical state that readily conforms to an interface surface of the substrate, e.g., being in the form of a semi-plastic material or a liquid slurry. The conformable material mixture is applied to the an interface surface of the cutting element to provide a green state material layer thereon. Depending on the particular application, the material mixture can be applied in the form of a coating onto the interface surface or, prior to application, can be preformed into a part shaped to fit over the interface surface, which is later applied over the interface surface.
The applied material mixture is pressurized under conditions of elevated temperature to consolidate and sinter the material mixture, thereby forming the wear resistant surface. The material mixture is consolidated and sintered in a manner that avoids unwanted material migration between the applied material mixture and substrate, thereby providing a fully-densified wear surface having the desired properties of hardness and/or fracture toughness.